1. Field of the Invention
This invention relates to windings for electric motors and other electromagnetic machines, of the kind which are located in slots in an armature or other laminated core. This invention is particularly but not exclusively applicable to such windings used in the slots of laminated cores of linear induction motors.
2. Review of the Art
The conditions under which such windings have to operate in a linear induction motor, especially one utilized for traction purposes, are particularly rigorous. The laminated core in which the slots are found is planar rather than cylindrical, and thus tends to be significantly less rigid than conventional cylindrical armatures. The core and its associated coils must operate in a severe environment between a vehicle to be propelled and the induction plate with respect to which the armature reacts, and are thus directly exposed to dirt, moisture and widely varying temperatures. Core assemblies constructed using conventional techniques have often proved to have an unacceptably short life under such conditions, leading to frequent failures and high maintenance costs.
Conventionally, the prepared and insulated windings are potted into the slots utilizing an epoxy resin compound, but it is found that under the stresses to which they are subjected in the cores of linear induction traction motors, such compounds are prone to microcracking followed by insulation failure as moisture penetrates to the windings. Other failure modes involve moisture penetration at other points of the windings, particularly at the external connections to the windings where the insulation is both highly stressed and vulnerable to moisture penetration at the interface with the connection assembly.
Various proposals have been made for providing improved protection of windings in slots in the cores of electrical machines.
Canadian Patent No. 638,999 (Brush) discloses a technique for producing water-tight windings in the slots of electrical machines in which the slots are lined by a sheet of silicone rubber sandwiched between two insulating sheets serving to protect the rubber, laying the winding within the lining, introducing a silicone paste into the slot, folding the lining over the winding, plugging the entrance to the slot, and curing the silicone rubber. This technique has the disadvantage that the coils must be formed and the rubber cured in situ, and that the coils can only be tested as part of the completed assembly.
Canadian Patent No. 752,551 (Dow Corning) discloses a technique for insulating high voltage windings using lapped silicon rubber tapes wound over an inner layer of silicone paste.
Canadian Patent No. 810,373 (General Motors) discloses an armature sealing technique in which resin coated conductors forming the windings are seated in resin coated slots in an armature, and portions of the windings outside of the slots are then wrapped in successive layers of silicone rubber, polytetrafluoroethylene sheeting, and glass-backed silicone rubber banding.
Canadian Patent No. 1,065,942 (General Electric Company) discusses problems encountered arising from the deterioration of epoxy resin impregnated windings, and seeks to overcome these problems by introducing silicone rubber bonded mats between the windings and the slots.
Canadian Patent No. 1,225,113 (General Electric Company) discloses slot armor for lining armature slots, in the form of a rigid glass fibre reinforced laminate with external layers of polyimide resin covered in turn by a fluorocarbon lubricating layer to reduce chafing.